Technical Abstract:
Understanding the molecular basis behind tree architecture could have significant impacts in tree crop agriculture and forestry. The ability to manipulate tree branch growth and patterning could lead to significant productivity improvements through reduced pesticide use, reduced labor for harvesting and pruning, and mechanization. In general, branch development and growth is directed by hormone gradients and influenced by gravity and light perception. However, many of the specific mechanisms responsible for the underlying traits behind branch architecture, such as the control of branch angle, remain unknown.
We are studying two prominent single locus traits associated with branch architecture in peach. The first is the pillar growth habit (also known as broomy or columnar), where tree shoots all have very narrow branch angles and grow vertically. Using a sequencing-based mapping approach, we call pnome mapping (for pooled genome), the peach gene responsible for the pillar growth was identified as a homologue of the Tillar Angle Control 1 (TAC1) gene from rice. Bioinformatic analysis revealed that TAC1 is a member of a novel IGT family of proteins, and that TAC1 genes evolved in vascular plants from another IGT protein, LAZY1. Rice and arabidopsis with null mutations in LAZY1 have wide tiller or branch angles, respectively. LAZY1 mutants lack gravitropism responses and show impaired polar auxin transport. The opposing phenotypes of TAC1 and LAZY1 mutants and their phylogenic relationship suggest that these two genes function together or in competition with one another to control branch angle development.
The second growth habit we are studying is the weeping phenotype, where branches turn and grow downwards. Interestingly, pillar (TAC1) was shown to be epistatic to the weeping growth habit phenotype. Trees which are homozygous for both pillar and weeping loci show only the pillar phenotype. Additionally, trees that are homozygous pillar but heterozygous for the weeping allele have an intermediate architecture phenotype called “archer”. These genetic interactions suggest TAC1 and the weeping gene product may be involved in the same developmental pathway responsible for branch angle initiation. Preliminary pnome sequence-based mapping has pinpointed the weeping locus to a 2 Mb region of chromosome 3. RNA-seq analysis of pillar, weeping, and archer apical meristems is being used to clarify the connection between these genes to each other, to overall and branch angle development, and to LAZY1. Future identification of the gene responsible for the weeping phenotype along with detailed auxin localization analysis in these trees and their corresponding arabidopsis mutants will also help elucidate the mechanism behind branch angle initiation.